Common industrial methods for producing styrene typically include separation and purification processes such as distillation to remove unwanted impurities. Unfortunately, purification processes carried out at elevated temperatures result in an increased rate of undesired polymerization. Distillation is generally carried out under vacuum to minimize loss of monomer. The presence of oxygen, which is typically excluded from styrene distillation, will also promote polymerization of the monomer.
This polymerization results not only in loss of desired monomer end-product, but also in the uses of production efficiency caused by polymer formation and/or agglomeration of polymer on process equipment. Thermal polymerization, which typically occurs during distillation, of styrene monomer results in the formation of normal (i.e., linear) polymer. This resulting polystyrene polymer is characterized by its glassy and transparent appearance and its solubility in the styrene monomer and many organic solvents.
The compounds generally used commercially to inhibit polymerization of vinyl aromatic monomers are of the dinitrophenol family. For example, U.S. Pat. No. 4,105,506, Watson et al. teaches the use of 2,6-dinitro-p-cresol as a polymerization inhibitor for vinyl aromatic compounds. U.S. Pat. No. 4,466,905, Butler et al. teaches that a combination of 2,6-dinitro-p-cresol and p-phenylenediamine will inhibit polymerization in a distillation column when oxygen is present. U.S. Pat. No. 4,774,374, Abruscato et al. teaches compositions for inhibiting the polymerization of vinyl aromatic compounds. The composition is an oxygenated product of the reaction of N-aryl-N'-alkyl-p-phenylenediamine with oxygen. U.S. Pat. Nos. 5,426,257 and 5,489,718, Arhancet, teach methods and compositions for inhibiting the polymerization of vinyl aromatic monomers comprising an oxime compound and a hydroxylamine compound and/or a phenylenediamine.
U.S. Pat. No. 5,254,760, Winter et al. teaches compositions and processes for inhibiting vinyl aromatic compound polymerization. The processes comprise adding to the vinyl aromatic compounds during purification or distillation a mixture of a stable hindered nitroxyl compound and an aromatic nitro compound. U.S. Pat. No. 4,670,131, Ferrell, teaches methods for inhibiting fouling of hydrocarbons containing unsaturated compounds with a stable free radical. The stable free radical can be a nitroxide compound.
A variety of inhibitor compositions have been employed in styrene and other vinyl aromatic monomers to inhibit undesired polymerization. Agents that have been used include sulfur, p-benzoquinone, tert-butyl pyrocatechol, phenothiazine, and hindered phenols. However, many of these compounds present disadvantages such as high intoxicity, instability, explosive hazard at elevated temperature and insufficient efficacy under processing conditions (i.e., inhibitor requires oxygen to be effective). The present inventor has discovered a novel composition which acts to inhibit vinyl aromatic monomer polymerization while avoiding the problems of certain known inhibitors.